This invention relates to control systems in which control is primarily or wholly effected by signals from digital computers. Such systems are commonly referred to as "full authority" digital control systems. In such control systems the control activity is commonly regulated by a digital computer which is itself responsive to sensed and/or desired operating conditions of an apparatus to be controlled. The number of these operating conditions may be large, and each of the individual conditions may fall within substantial ranges of values. The combinations and sequences of the operating values will then be very large indeed, so that it will not be possible to test the system over the whole of the combined operating conditions which it may encounter in use. The problem is increased by the relative ease with which a digital control system may be reprogrammed, so that extensive tests which may have been carried out with the system under control of a superseded programme will no longer be valid.
Inability to predict the response of a full authority system under all operating conditions has lead to reluctance on the part of authorities connected with air-worthiness to approve such system for use in an aircraft, particularly since an inappropriate response of the system to a combination of conditions, resulting for example from a programming fault, will not occur randomly but may, in that combination of conditions, cause malfunction of all apparatus controlled by the digital computer. It will be apparent moreover that duplication of identically programmed processors will not overcome this problem, since each malfunction simultaneously and in the same way. The present invention provides a full authority digital control system in which the foregoing problems are overcome.